1. Field of the Invention
The present invention relates to a flip chip assembly process and a tool therefor, and more specifically to a process and a tool to form an encapsulant on a substrate by screen printing.
2. Description of the Related Art
Due to the demand for high-density and high-power electronic packaging, flip chip technology has become important in many fields. Flip chip technology is characterized by flipping over the bare die for attachment to a substrate. When an organic material is used as the substrate, the great difference between the coefficient of thermal expansion between the organic substrate (14-17 ppm/xc2x0 C.) and the silicon wafer (4 ppm/xc2x0 C.) causes significant strain on the solder connections during temperature cycling, which results in easy deterioration of the connections.
Therefore, to enhance reliability, encapsulant is usually filled into the space between the substrate and the chip. In this way, the stress can be dispersed to the encapsulant to decrease the stress on the connections. Thus, connection cracking can be alleviated, the crack will not extend, and the life of the connections is prolonged. In addition, the encapsulant can also prevent the transmission of leak current caused by impurities between the solder connections. Statistical data shows that the reliability of the chip can be increased five to ten times once underfill encapsulation is conducted. Therefore, underfill encapsulation is a necessary process. However, it suffers from the long time required for underfilling and curing of the encapsulant.
Currently, most flip chip packages are encapsulated by dispensing a liquid encapsulant with low viscosity along the periphery of the chip. Capillary action, generated from the encapsulant in the fine space (less than 100 xcexcm) between the chip and the substrate, drives the encapsulant to fill the gap between the solder connections. Since filling is conducted by capillary action, it is very slow. This problem becomes even more serious as the chip size increases because filling time is proportional to the square of the length of the chip. For example, in a typical encapsulation operation, the filling takes several minutes to several tens of minutes for a 7 mm square chip depending on the filling temperature. As well, since capillary action is insufficient and the pressure cannot be effectively maintained, voids are easily formed in the encapsulant when the filling is complete. Moreover, the voids cause the flip chip package to be scrapped from either popcorn effect, caused during the following thermal processes, or stress concentration, caused when the flip chip package is stressed. Thus, reliability is adversely affected. Also, one set of underfill tools can only encapsulate a single flip chip package at a time, so the only way to increase underfill throughput is to purchase more underfill tools, increasing production costs. Finally, it is necessary to divide the substrate into single packages before filling the encapsulant, causing low throughput in subsequent process such as ball placement.
Thus, the main object of the present invention is to provide a flip chip assembly process and a tool therefor allowing underfill to be finished in a shorter period to increase the underfill throughput of flip chip assembly.
Another object of the present invention is to provide a flip chip assembly process and a tool therefor that prevents voids in the encapsulant, improving the production yield of flip chip assembly and to produce a flip chip package with higher reliability and longer life.
Another object of the present invention is to provide a flip chip assembly process and a tool therefor allowing a plurality of divided substrates respectively having one single package unit to be encapsulated at the same time to further increase the underfill throughput of flip chip assembly.
Another object of the present invention is to provide a flip chip assembly process and a tool therefor allowing one substrate having a plurality of package units, or a plurality of substrates respectively having a plurality of package units to be encapsulated at the same time to further increase the underfill throughput of flip chip assembly.
In order to achieve the above objects, the present invention provides a flip chip assembly process forming an underfill encapsulant. First, a chip comprising an active surface, having a plurality of conductive bumps arranged in array with a predetermined pitch, is provided. Then, a substrate comprising a surface, having a die-attaching region, having a plurality of pads with previously formed solder paste on each, arranged in array having a predetermined pitch, is provided. Further, an encapsulant is formed in the die-attaching region excluding the pads, using a stencil and screen printing. Finally, the chip is attached to the substrate by one-to-one joining of the conductive bumps and the pads.
The present invention further provides a tool forming an underfill encapsulant, comprising a printing region and other regions. The printing region further comprises a plurality of openings, a plurality of covers, and a plurality of connecting devices connecting one cover with the neighboring stencil region.